Acid Rain Measurement

In the 1970s, we realized the importance of monitoring the acidity of rainfall. The National Atmospheric Deposition Program (NADP) was formed in 1977 to investigate and measure acid deposition. There were 22 stations nationwide in 1978. Currently, the National Trends Network, which now conducts measurements for NADP, has nearly 220 stations. Each station collects a sample once a week. Samples are collected in a container. This container is covered until rain begins. The presence of water automatically opens the container, and it remains open until the rain stops. These samples are analyzed and sent to a central location. The data go into a database which now stores data from over 20 years, and gives us a feel for how acidity varies with location, and also the trend over this period. This picture above shows a typical acid rain monitor. The container on the left represents an attempt to measure dry deposition. When precipitation begins, a sensor causes the cover to move from the right container to the left. Rain water will then collect in the right container.

Dry Deposition is difficult to measure directly. There have been attempts to use a method similar to that for wet deposition, but it usually takes too long to cover the collector when rain starts. It takes only a few drops of rain to contaminate a dry deposition sample. The Clean Air Status and Trends Network (CASTNet) calculates dry deposition using atmospheric conditions, meteorological data, and information on land use, vegetation, and surface conditions.

Other programs have looked at changes in the acidity of lakes, streams, and soil. Monitored data for some lakes are available from as early as 1980. Facilities such as Hubbard Brook in New Hampshire have been measuring the acidity of streams and lakes on their 7800-acre property. They also have looked at soil chemistry and effects on the entire ecosystem.

In addition to specific efforts to measure acid rain, EPA maintains monitoring networks for monitoring SO2 and NO2 in the air for other programs. Although these program are not intended to consider acid rain, they do give us an idea of the amount of these chemicals in the air at certain locations.